1. Field of the Invention
The present invention relates to a planar illumination device.
2. Description of the Related Art
Today, liquid crystal display apparatuses used in combination with planar illumination devices enjoy a widespread use as display devices for personal computers, cellular phones, and other electronic apparatuses because of their advantage of ease of reduction in thickness. With the recent increasing trend toward greater performance offered by white light emitting diodes (LEDs), planar illumination devices incorporating white LEDs as light sources are becoming more common, which represents industry's efforts toward achieving even further reduction in size and thickness and even lower power consumption of the planar illumination devices. Such a planar illumination device includes, a light guide plate having a pair of principal surfaces opposed to each other, in which one of the principal surfaces serves as a light emitting surface, a LED disposed to face a light incident surface of the light guide plate, and a circuit substrate on which the LED is mounted. A white LED 100 illustrated in FIG. 3, for example, is used as the LED (see, for example, Japanese Patent Application Laid-open No. 2014-164995). The white LED 100 includes a blue LED chip 102 encapsulated by a yellow phosphor 104 in a lamp house 106 formed of a white resin. As illustrated in FIG. 3, the blue LED chip 102 is disposed at a substantially central portion in a longitudinal direction of the lamp house 106. Additionally, an electrode terminal (not illustrated) is disposed at each of both ends in the longitudinal direction of the lamp house 106. Flexible printed circuit boards (FPCs) are widely used as the circuit substrate on which the electrode terminals are mounted.
The FPC has a coverlay film that protects a conductor layer. A side-light type planar illumination device including a white coverlay film has been developed (see, for example, Japanese Patent Application Laid-open No. 2013-131444). The white coverlay film has a white reflective layer formed on an entire surface of a film substrate, functioning as a protective member and a light reflecting member. Specifically, even in a display device structure involving a gap between a light emitting surface of the LED and a light incident surface of the light guide plate, the white coverlay film blocks this gap to thereby reduce an amount of light that leaks from this gap to the outside, in which the light is emitted from the light emitting surface of the LED. Additionally, light reflected by the white coverlay film is incident upon the light guide plate through the light incident surface of the light guide plate, so that luminance of the light emitting surface of the light guide plate can be improved.
When the white LED 100 composed of the blue LED chip 102 and the yellow phosphor 104 as illustrated in FIG. 3 is used as a point light source of a planar illumination device, light rays L2 and L1 emitted in directions inclined with respect to an optical axis tend to be more tinged with yellow than with a light ray L3 emitted in a direction parallel with the optical axis. Moreover, the light L1 that is angled more greatly from the optical axis is more tinged with yellow than the light ray L2. This light being tinged is attributable to a distance over which the light emitted from the blue LED chip 102 travels before reaching a light emitting surface 100a, specifically, a light path through the yellow phosphor 104 being longer in the following order: L3<L2<L1. In addition, the light L1 and the light ray L2 that are inclined with respect to the optical axis are incident upon the light incident surface of the light guide plate and tend to be emitted from a region of the light emitting surface on a side closer to the light incident surface. Thus, when the white coverlay film is used for the coverlay film of the FPC, light emitted from the region of light emitting surface of the light guide plate on the side closer to the light incident surface of the light guide plate is tinged with yellow, sometimes resulting in uneven color being perceived.
Meanwhile, a need exists in the planar illumination device for a thinner configuration, in addition to a need for improved illumination performance; however, as the configuration becomes thinner, the uneven color tends to occur more conspicuously.